<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    .box:first-child {
      color: pink;
    }
    .box:last-child {
      color: blue;
    }
    .box:nth-child(2) {
      color: green;
    }
  </style>
</head>
<body>
  <div class="box" id="first">我是第一个box</div>
  <div class="box" id="first">我是第二个box</div>
  <div class="box" id="first">我是第三个box</div>
</body>
</html>